Understanding Climate: Biogenic Silica As a Proxy for Interglacial and Glacial Periods 3 to 5 Million Years Ago

Short Contributions 29th Annual Symposium Volume 23rd April, 2016 ISBN: 1528-7491 Published by Keck Geology Consortium

UNDERSTANDING CLIMATE: BIOGENIC SILICA AS A PROXY FOR INTERGLACIAL AND GLACIAL PERIODS 3 TO 5 MILLION YEARS AGO

CINDY EUNICE FLORES, Wesleyan University Research Advisor: Suzanne O’Connell

ABSTRACT and sponge spicules can also contribute. Diatoms and radiolarians are both single cellular organisms Paleoclimatology, the study of Earth’s past climate, that live at varying depths in the water column. allows us to determine how Earth systems responded Diatoms are algal photo synthesizers and radiolarians to different climatic conditions. This field is are zooplankton that feed on other single cellular particularly relevant due to its potential applications organisms in an amoeba like fashion. As these to current global climate change. As Earth’s climate organisms grow they make their skeleton from silica continues to warm, areas of high latitude such as (SiO2) (Demaster 1981). When the organism dies it Antarctica experience the most warming, drawing sinks to the bottom of the ocean and its silica skeleton concern to ice melt and consequent sea level rise. The is deposited in the marine sediment below. dominant contributor to current climate warming is the increase in atmospheric CO2, which now exceeds It should be noted that the deposition of silica in 400 ppm. During parts of the Pliocene Epoch, 5.3 marine sediment is quite rare. The reason being that to 2.6 million years ago, atmospheric CO2 is also the earth’s oceans are under saturated in silica so the thought to have exceeded 400 ppm. One control on biosilica is dissolved by the ocean water. The only atmospheric CO2 levels is the amount of carbon that exceptions to this are areas of high productivity where can be sequestered by down welling and biological there are so many diatoms and radiolarians that some productivity, especially in the Southern Ocean. of their silica skeletons reach the seafloor and are Through biosilica analysis of sediment cores from buried. The Antarctic is an area of high productivity Site 697 from the northwestern Weddell Sea north of due to the upwelling of nutrients and therefore our the Antarctica Peninsula, we seek to understand when site in the Weddell Sea in Antarctica has a record of during glacial and interglacial periods productivity is biosilica. highest. We hypothesize that high biosilica content corresponds to increased productivity and to warm Microfossils can be used as proxies for climate. When interglacial periods. Through this study, we attempt identified by species, diatoms can tell us about the to establish a more detailed understanding of the range of oceanic temperatures because every species environmental changes likely to occur in the earth’s has specific parameters for survival (see Robakiewicz, climate and the Antarctic ice sheet’s response. this volume). The presence of diatoms themselves tells us one extremely important fact; ice in the form of INTRODUCTION permanent sheets must not have been present. Diatoms cannot photosynthesize under ice sheets, although Biosilica refers to marine sediments that are formed some live in and on ice sheets. If they are present in through biological processes. The main contributors the sediment record, ice sheets must not have extended to biosilica in today’s oceans are diatoms and to this area of the Weddell Sea. By examining the radiolarians, although silicoflagellates, discoasters fluctuations in diatom levels we can begin to establish 1 29th Annual Symposium Volume, 23rd April, 2016 a pattern of warm interglacial periods with high sample so that you have 20 caps of sediment per diatom abundance and cooler glacial periods with 10 unique samples. little to no diatom presence. 8. Three of the remaining caps were left empty to The record of biosilica we examine in this study dates serve as operational blanks. The other cap was back 3 to 5 million. The dynamic past of the Antarctic filled with 25 mg of a laboratory biosilica standard ice sheets is unknown and given the current pattern made from extra sediment from the core. of global warming it is highly relevant to study the response of these ice sheets to high levels of carbon Part 2: Digestion dioxide in the past. 1. Set a hot water bath to 85° C. Use a rack to place METHODS the 50 mL tubes of NaOH inside the water bath. The biosilica analysis procedure was adapted from the 2. Once up to temperature begin digestion National Lacustrine Core Facility of the University by dropping each cap of sediment into its of Minnesota. Below is a brief summary of the three- corresponding NaOH tube. Make sure to record part process. All samples for this study were collected your starting time. You have 30 seconds to deposit from a marine sediment core drilled on ODP Leg 113 each cap of sediment into each tube and shake the Hole 697B. tube.

Part 1: Initial Preparation 3. Repeat step 2 for each sample.

1. Freeze dry sediment samples overnight to remove 4. Take your recorded start time and calculate when any moisture. 60, 90, 120, 150, 180, and 200 minutes will have passed. 1 mL extractions of the NaOH will be 2. Take a portion of each dried sample and grind into taken at 60, 90, 120, and 200 minutes. At 150 and a fine powder with a mortar and pestle. Store each 180 minutes each sample will be shaken for 30 unique sample in a labeled one-dram vial. seconds to suspend sediment.

3. Make all reagents according to the guidelines in 5. After 60 minutes have passed use a pipette to LabCore manual with the following exception: use extract 1 mL aliquot from each NaOH tube and 40 g of NaOH to make one molar NaOH instead of deposit it in its corresponding 15 mL tube with 0.5 molar. double deionized water. Change tips every time you extract an aliquot. You have 30 seconds to 4. Make four sets of 15 mL tubes with twenty-four collect an aliquot for each of the 24 digestion tubes per set. Label the caps of these tubes A1- tubes. C8. Additionally label your sets as 60 minutes, 90 minutes, 120 minutes, and 200 minutes. 6. Repeat step 5 for each of the extraction times.

5. Use a pipette to fill each 15mL tube with 4 mL of Part 3: Analysis double deionized water. 1. Turn on spectrophotometer and set to read 860 nm 6. Label twenty-four 50 mL tube caps from A1 to C8. wavelength. Use a pipette to fill each tube with 38 mL of one molar NaOH. 2. Set up four sets of 10 mL plastic beakers corresponding with your set of aliquots for 60, 90, 7. Select 10 of one-dram samples to analyze. Label 120 and 200 minutes. Additionally, make a set of 20 one-dram caps A1-C8. Weigh out 25 mg of 10 mL beakers for your silica standards to create each sample into the labeled one-dram caps and your absorbance curve. record weights. Make sure to duplicate each 2 29th Annual Symposium Volume, 23rd April, 2016

3. Pipette 2 mL of Molybdate solution into each 8. After enough time has passed, run double plastic beaker. deionized water through the spectrophotometer.

4. Pipette 0.5 mL of each aliquot into its 9. Start running samples through the corresponding plastic beaker. Change pipettes for spectrophotometer beginning with your standards, each sample. Record the time you pipette the last then 60, 90, 120, and 200-minute sets. Record the sample. spectrophotometer values. (Best done on the same spreadsheet data where you recorded the weight of 5. Dispense 4 mL of double deionized water into your samples. each of the beakers within 15 minutes of pipetting the last aliquot. 10. Run double deionized water through the spectrophotometer between each sample. 6. Dispense 3 mL of reducing solution into each beaker within 15 minutes of pipetting the last of 11. When all samples are finished run several the double deionized water into beakers. rounds of double deionized water through the spectrophotometer and turn it off. 7. Wait at least 3 hours for color of the samples to fully develop. Do not wait more than 5 hours to For instructions on how to set up the spread sheet see analyze. the LabCore manual.

Table 1. Here is an example of our biosilica data set. Standards made from known concentrations of biosilica make up our standard curve used to calculate the Weight % of Biosilica present in each sample. Each sample has its own ID and four aliquots. Every unique sample was run twice and the average of the two values was used as the actual % Biosilica for our data.

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RESULTS The overall goal of this project was to investigate if change in biosilica followed other proxy patterns of change in the Weddell Sea. We hypothesized: 1. High levels of biosilica would correlate with warmer temperatures and high IRD accumulation from melting icebergs. 2. The presence of chert in some samples might be correlated with high biosilica weight % values.

A total of 126 samples were analyzed for biosilica percentage and plotted versus time. The samples range from 1.30% to 13.35% percent abundance of biosilica.

Figure 2. Side by side graph comparison of biosilica percentage, accumulation of coarse fraction over time, and diatom content by percent. Notice the slightly delayed peaks of all three components and their synchronization at several time intervals notably at from C2AN-3 to Gauss. (See Cullen in this volume.)

Figure 1. Part A depicts the change in biosilica percentage for Site 697 Hole B cores 13X-17X. The red lines are ages from Part B. Part B magnetic reversal data comes from Hamilton and O’Brian (in prep ) found in Pudsey 1990. The current ages come from Gee and Kent 2007. Figure 4. The cross plot above shows that there is no correlation When put side by side with a record of coarse between levels of IRD and Biosilica. The highest values of fraction accumulation we see a similar pattern in biosilica coincide with the lowest values of IRD, which is their amplitude and synchronization at certain time counterintuitive to our original hypothesis. intervals. This indicates that there must be some force driving change in both of these systems.

However, when we plotted our IRD levels with our Biosilica data in a cross plot we see that there is no correlation. In fact, the highest values of biosilica correspond with the lowest levels of IRD and the lowest values of biosilica corresponded with the highest IRD values. Biosilica is therefore independent of IRD, disproving our initial hypothesis that high levels of IRD would correlate with high levels of 4 29th Annual Symposium Volume, 23rd April, 2016

Biosilica because icebergs would indicate the melting REFERENCES of ice sheets. http://lrc.geo.umn.edu/laccore/assets/pdf/sops/ Our second hypothesis also proved to be false. The Biogenic%20Silica%20SOP%202011.pdf presence of chert, which is a mineral composed of Demaster, David J. “The Supply and Accumulation of silica that has been warmed and compressed, in a Silica in the Marine Environment.” Geochimica sample has no correlation with the amount of Biosilica Et Cosmochimica Acta 45.10 (1981): 1715-732. Weight %. Web. Pudsey, C.j. “Grain Size and Diatom Content of Hemipelagic Sediments at Site 697|ODP Leg 113: A Record of Pliocene-Pleistocene Climate.”Proceedings of the Ocean Drilling Program, 113 Scientific Reports Proceedings of the Ocean Drilling Program (1990). Web. Gee, J.s., and D.v. Kent. “Source of Oceanic Magnetic Anomalies and the Geomagnetic Polarity Timescale.” Treatise on Geophysics (2007): 455- 507. Web.

Figure 3. These figures represent the relationship between Biosilica Weight % and the presence of the mineral chert our samples (see Cullen in this volume). Part A is the fraction of samples with chert present and their corresponding Biosilica Weight % values. Part B shows samples without chert and their Biosilica Weight %. Neither of these graphs show no significant correlation.